As is well known in the art, the successful implementation of optical communications systems requires devices capable of reliably controlling the propagation of light. Basic optical processing functions required in optical communications include, but are not limited to: optical modulation (which includes amplification and attenuation of optical power); phase control (delay); and switching. In Wavelength Division Multiplexed (WDM) and Dense Wavelength Division Multiplexed (DWDM) communications systems, significant channel power imbalance may be generated and channel equalization become necessary where the above noted basic functions must be performed on a per-channel basis.
In modern high performance optical communications systems, data rates of 10 GHz or more can be encountered on each channel. In addition, using known optical amplification techniques such as Raman pumping and Erbium Doped Fiber Amplification (EDFA), optical transmission spans of 1000 Km or more can readily be achieved.
In general, there are two broad classes of known optical processing devices; namely Out-of-Fiber and In-Fiber. Out-of-Fiber devices typically involve extracting the light out of the transmission fiber and into a system of micro-optics. Within the micro-optical system, passive elements such as mirrors and lenses are combined with active elements such as liquid crystal arrays and/or Micro-Electro-Mechanical Systems (MEMS) to perform a wide range of sophisticated optical control functions. Light emerging from the micro-optical system is then coupled back into the transmission fiber to continue toward its destination.
Out-of-Fiber systems suffer numerous disadvantages, including difficulties manufacturing the micro-optical system components, stability of the system during service, and the high optical losses imposed by such systems. Losses are encountered within the micro-optical system, and more than 1 dB total loss will be encountered when coupling the light back into the transmission fiber. As a result, Out-of-Fiber systems typically require more optical amplifiers, which further increases the cost of the system.
In principle, many of the difficulties that are associated with Out-of-Fiber systems could be overcome by In-Fiber systems, in which light propagating within the fiber is controlled without removing the light from the fiber. One of the earliest In-Fiber systems involved optical amplification, in which stimulated Brillouin scattering is used to amplify optical signals propagating within a fiber. Other known devices utilize the reflective and transmissive properties of fiber Bragg gratings (FBG) and long period gratings (LPG), respectively, in the UV radiation sensitive core of fibers. See, for example, U.S. Pat. No. 4,474,427 (Hill et al.; U.S. Pat. No. 5,912,999 (Brennan et al.).
FBGs are spectral filters, which typically reflect light over a narrow wavelength range and transmit all other wavelengths. They also can be designed to have more complex spectral responses. Such filters were widely used in sensing and optical communications, such as add/drop filters, dispersion compensators, spectrum equalizers, etc.
Initially, FBG and LPG based devices were fabricated mainly for static operation. However, the performance of many optical components is frequently affected by environmental conditions and dynamic network configuration changes, and thus is strongly time varying. This requires the design and fabrication of dynamically controllable devices, especially wavelength selective components, such as tunable filters and variable attenuators. Some of the presently known solutions use FBGs and LPGs. For example, different ways have been proposed to dynamically change the resonant operation conditions of FBGs. These methods are based on the change of the effective refractive index of the core   n  eff  coreor the geometrical period ΛFBG of the grating (the resonant Bragg reflection wavelength being       λ    R    =      2    ⁢          n      eff      core        ⁢          Λ      FGB      ). The known methods use thermo-optic, piezoelectric, acousto-optic effects or fiber stressing mechanisms.
For example, U.S. Pat. No. 5,007,705 (Morey et al.) teaches a tunable FBG in which a heating electrode is used to change the geometrical period ΛFBG of the grating or the effective refractive index   n  eff  coreof the core material. U.S. Pat. No. 5,699,468 (Farries et al) teaches a FBG based variable optical attenuator (filter) in which a piezo-electric transducer is coupled to each FBG. When a transducer is energized, it compresses or expands the respective grating, thereby changing the grating period and thus its reflection wavelength. Both of these devices are highly temperature sensitive and power consuming, both of which are undesirable, particularly in compact integrated geometries.
U.S. Pat. Nos. 5,966,493 and 6,370,312 (both to Wagoner et al.) teach tunable optical attenuators in which the cladding of an optical fiber is side-polished to expose a surface though which light propagating in the fiber core can escape. A controllable refractive index material is positioned against this surface. Changes in the effective refractive index of the controllable index material can be used to control the amount of light coupled out of the fiber core U.S. Pat. No. 6,011,881 (Moslehi et al.) teaches a tunable optical filter in which the cladding of an optical fiber is side-polished in the vicinity of a FBG. A controllable refractive index material is positioned against this surface. Changes in the refractive index of the controllable material can be used to vary the refractive index   n  eff  coreof the core material, and thus the reflective wavelength of the FBG. Because these devices require a side-polished fiber, the optical properties of which are also highly dependent on the radius of curvature and the distance between the polished surface and the fiber core, they tend to be difficult to manufacture. They also tend to be highly sensitive to temperature. Finally, because of the curvature and asymmetry of the side-polished fiber, its optical performance may be polarization dependent, which in many cases is undesirable.
There is a different situation (as compared to FBG) for LPG based tunable devices. In the case of LPG devices, light may be out-coupled from the fiber core and propagate in the cladding of the fiber, the properties of which are easier to change using an external controllable material. The resonant condition of light coupling between the core and the cladding is a function of the difference of the effective refractive index of the core mode   n  eff  coreand the effective refractive indices of the cladding modes       n    eff    clad    ,and also of the period of the LPG ΛLPG:       λ    R    =            (                        n          eff          core                -                  n          eff          clad                    )        ⁢          Λ      LPG      Thus U.S. Pat. No. 6,058,226 (Starodubov) teaches an optical system for selectively filtering and modulating light extracted from the core to the cladding area by means of a LPG. This energy transfer is produced by a resonant (and thus wavelength-selective) mode coupling process between the co-propagating fundamental core and higher order cladding modes of the fiber. Different gratings and electrically driven elements can be combined to provide various types of filters, sensors, modulators and delay lines. The basic element of these different configurations is an electrically sensitive material which is disposed surrounding the cladding. The application (across the electrically sensitive material) of a voltage causes the refractive index of the material to change. This causes a change of the effective refractive index of the cladding       n    eff    clad    ,which in turn influences the propagation characteristics of cladding modes. This in turn changes the resonant coupling condition between the cladding and fundamental core modes. Finally, as this process is wavelength sensitive, it changes the spectral transmittance characteristics of this LPG based device.
However, in contrast to FBG-based devices, the period ΛLPG and spectral bandwidth ΔλLPG of LPGs are typically very large (up to 1000 times larger than for narrowband FBG) and are not well adapted for narrow band (e.g., single WDM channel) applications. Note that Starodubov teaches a device in which a passive (non-tunable) FBG is used in combination with two tunable LPGs. Thus the electrical modulation (tuning) of the signal frequency still remains quasi broadband, since the radiation is extracted from the core by a quasi-broadband tunable LPG filter. In addition, several transfers are performed here between the core and the co-propagating cladding modes. Finally, Starodubov uses electrodes deposited directly on the cladding of the fiber since otherwise, the electrodes on opposed sides of the fiber would be too far apart (more than 125 μm), thereby requiring thus much higher voltages to induce refractive index changes. However, this solution also is very limited since it inevitably introduces losses of light because of the complex refractive index of the material forming the electrode. These conditions limit the application of that device as an efficient, low loss and narrowband-tuning element. At the same time, as mentioned above, the tunable FBG based filters could be useful for such applications. However, the thermal and mechanical methods, which affect the core zone of the fiber, are not efficient.
Accordingly, efficient In-Fiber optical devices remain highly desirable.